1. Field of the Invention
The present invention relates to an antirust treatment method for an aluminum die-cast part, and an aluminum die-cast part for a vehicular lighting fixture. More particularly, it relates to a technique for performing an antirust treatment method for an aluminum die-cast part for a vehicular lighting fixture at a low cost without performing a chromate treatment or a zirconium treatment.
2. Description of the Related Art
Conventionally, from the viewpoint of heat resistance and light distribution control accuracy, an aluminum die-cast part has been used in a vehicular lighting fixture. One example of the aluminum die-cast part for a vehicular lighting fixture is a shade that is disposed in a projector type headlamp and has a reflective surface for controlling overhead light distribution illuminating traffic signs while forming a cutoff line for low beam.
FIG. 5 is a flowchart for explaining a general manufacturing process for a conventional shade.
As shown in FIG. 5, the conventional shade is manufactured through the following steps: a step of injection molding an aluminum alloy (an alloy for die-casting specified by JIS symbol ADC12) that contains silicon by using a die-casting machine (Step S100); a step of deburring (blasting etc.) the injection molded shade (Step S101); a step of degreasing the deburred shade with a detergent solvent to remove fats and oils (Step S102); a water washing step (Step S103); a step of applying an antirust treatment to the washed shade (chromate treatment (for example, Japanese Patent Application Laid-Open No. 2005-171296) or zirconium treatment (for example, Japanese Patent No. 3437023)) (Step S104); a water washing step (Step S105); a hot-water washing step (Step S106); a drying step (Step S107); and the like steps. Between the deburring step (Step S101) and the degreasing step (Step S102), a barrel polishing step (Step S108) is sometimes provided to increase the surface reflectance.